In legal documents you transliterate from foreign alphabets to the local one according to legal rules on how the transliteration should be performed. In this case, all the "incompatible" letters are a key part of the local language, which the bank is required to support - most of the world is not like USA which famously doesn't technically implement the concept of an official language.
But technically your point about transliteration is a valid solution (and one that I have actually seen in practice in banking systems) - the bank is free to transliterate the name to something that fits in ECBDIC, as long as it also then appropriately transliterates it back to use the proper name in the documents and communication. The law doesn't mandate any technical nuances of data storage, as long as they get the proper result.
But technically your point about transliteration is a valid solution (and one that I have actually seen in practice in banking systems) - the bank is free to transliterate the name to something that fits in ECBDIC, as long as it also then appropriately transliterates it back to use the proper name in the documents and communication. The law doesn't mandate any technical nuances of data storage, as long as they get the proper result.